1. Field of the Invention
The present invention relates to a recording apparatus capable of correcting an uneven image density produced due to variations in recording characteristics of a recording head having plural recording elements arranged therein and a correcting method of the nonuniformities in density. In particular, it relates to a technique of correcting the nonuniformities in density of a recording head in a record forming apparatus for forming an image by the recording head performing binary recording.
2. Description of the Related Art
Hitherto, image forming apparatuses for forming an image on a recording medium (hereinafter referred to as a recording medium, a recording sheet, or simply a sheet) have been proposed, which apparatuses have a recording head attached thereon and employ various recording systems therein. As the recording systems of the recording head, there are a wire dot system, a heat-sensitive system, a thermal transfer system, an ink-jet system, and so forth. In particular, the ink-jet system directly ejecting ink onto a recording sheet is inexpensive in running cost and is noted as a silent recording method.
In these various recording systems, gradation recording by recording elements arranged in the recording head is limited due to various reasons, so that an apparatus employing a binary recording system is also utilized. In particular, in the ink jet system, complicated control is required for controlling the size of an ejected ink drop and for modulating the size of an ink drop over a wide range, so that apparatuses employing the binary recording system which can be relatively easily controlled are widely used. As another image forming apparatus employing such a binary recording system (hereinafter referred to as a binary printer), there is known an electrophotographic printer using a recording head (referred to as an LED head) having LEDs (light emitted diodes) which are light emitting elements arranged therein. Also, it has been conventionally known that in the binary printer mentioned above, multi-level image data (hereinafter also referred to as an image signal) representing the gradation corresponding to density is converted into binary data by binarizing means so as to achieve gradation recording with area gradation by controlling recording through the binary data.
Hitherto, it has also been known that in such a binary printer, nonuniformities in density are produced in the recorded image due to variations in characteristics of an individual recording element of the plural elements arranged in the recording head.
In the ink-jet printer, for example, it is conventional that a recording head having plural nozzles arranged therein is used; ejecting means arranged in the recording head corresponding to each nozzle are driven; and ink drop ejection is controlled so as to perform binary recording. In such a structure, when variations in the amount of ink ejection from each nozzle in the recording head are produced, nonuniformities in density may be generated in recorded images. The reasons for the variations in the amount of ink ejection from each nozzle seem to be variations in shape and size of each nozzle of the recording head and changes in ink ejecting power by the ejecting means, among others. These reasons may frequently result from variations in the manufacturing process, so that the problem is difficult to be fundamentally resolved.
A bubble jet system is known among various ink-jet systems, in which a heater functioning as an electrothermal converter is adopted for generating thermal energy as ejecting means corresponding to an electric signal; bubbles are produced in ink by the thermal energy generated by the heater; and the ink is ejected by the pressure of the bubbles. In the bubble jet system, very small variations in thickness and area of the heater are produced in the manufacturing process, resulting in differences in the resistance value of each heater, so that variations in size of the ejected ink drop are generated and result in nonuniformities in density of the recording image.
As a correction technique for correcting such nonuniformities in density, a method called head shading is known. The head shading is a technique in which multi-level image data representing density corresponding to each nozzle are corrected. That is, the magnitude of the density represented by the image data is changed corresponding to dispersion in density of each nozzle so as to correct the nonuniformities in density, and thereby obtain an image having uniform density.
Also, in the printer using the LED head mentioned above, emitting dispersion of each LED arranged in the head may result in nonuniformities in density produced in the recorded image and the nonuniformities in density can be corrected by the head shading technique described above.
The head shading comprises the steps of: first, recording a pattern with a predetermined density using the recording head; then, reading the density of the recorded pattern; and correcting the density represented by the image data corresponding to each recording element based on the recorded density corresponding to each recording element.
In the conventional technique of the head shading, a correction table corresponding to each recording element is selected based on the result of the recorded pattern with a predetermined density. For example, in recording multi-level image data representing a density of 256 levels of gradation from 00H to FFH, if a recording element has recorded a pattern based on a density corresponding to the gradation value 80H, but is determined to have a density higher than the targeted ideal density, a table decreasing the density of the input image signal is set for that recording element. If a recording element is determined to record at a density lower than the ideal density, a table increasing the density of the input image signal is set for that recording element.
The procedure for table conversion will be described with reference to FIG. 10.
In FIG. 10, the abscissa indicates the density of the input image signal and the ordinate indicates the density of the image signal after conversion. A straight line A in FIG. 10 is a line having a gradient of 1, and in the conversion according to that line, the input image signal is output as it is with the same density without conversion of the density. A straight line B shows a table decreasing the output density of the input image signal and a straight line C shows a table increasing the output density of the input image signal.
Therefore, to the recording element recording at a density determined to be higher than the targeted density, such a table as indicated by the straight line B in FIG. 10 is applied, while to the recording element recording at a density determined to be lower than the targeted density, such a table as indicated by the straight line C in FIG. 10 is applied. In addition, the tables are not limited to the straight lines A, B, and C; it is possible to respond to nonuniformities in density of each recording element by preparing plural tables having lines with gradients different from those shown in FIG. 10. For example, 32 tables can be prepared as the conversion tables for the head shading, so that the nonuniformities in density can be satisfactorily corrected by applying any one of the 32 tables even when the range width of the nonuniformities in density for each recording element is large.
However, in the conversion table system shown in FIG. 10, with respect to density levels from 00H to FFH of the input image signal, the density is uniformly converted. That is, the conversion table of each recording element is set based on the result of the recorded pattern with a predetermined density level (the pattern of the density value 80H, for example). Consequently, a table uniformly increasing or uniformly decreasing the densities is applied to the input values from 00H to FFH of density. Therefore, a table performing correction over the entire density level range is set based on the result of the recorded pattern with a predetermined density level.
According to the conventional technique described above, for a recording element that records with density decreasing over the entire density level range or a recording element that records with density increasing over the entire density level range, i.e., for recording elements having the same density characteristics, nonuniformities in density can be efficiently corrected.
However, with respect to a recording element with density characteristics having a different correction effect (also referred to as correction sensitivity) depending on the density level, there has been a problem that normal correction cannot be performed at a specific density level. That is, when there is a recording element that records with density decreasing especially at a low density level, although correction is normally performed at the high density level, at the low density level, the density decreases even if correction is performed at the low density level, so that nonuniformities in density cannot be resolved because of density characteristics difference between the higher and lower density levels. In such a manner, when there is a recording element with density characteristics which differ according to the density level among recording elements of a recording head, nonuniformities in density also differ at a specific density level, so that there has been a problem that the density correction is not satisfactorily performed by the above-described technique.